Lithium metal phosphates represented by the chemical formula LiMPO4 and the like are known to be less expensive and to provide a safer battery than LiCoO2, and therefore are expected to serve as a cathode active material for use in a lithium-ion battery, in particular as a cathode active material for use in a large battery to be mounted on an automobile or the like.
Known methods for producing a lithium metal phosphate include solid phase synthesis, coprecipitation and calcination, glass crystallization, hydrothermal synthesis, and the like. Among these, hydrothermal synthesis is excellent because it can provide a lithium metal phosphate having a small particle diameter, which is suitable for a cathode active material for use in a lithium-ion battery.
Patent Document 1 discloses a method for producing lithium iron phosphate, and the method comprises placing lithium phosphate and iron (II) chloride tetrahydrate as well as distilled water in a pressure-resistant container, which is then subjected to substitution with argon gas, is hermetically sealed, and is heated in an oil bath at 180° C. to allow a reaction to proceed. Also disclosed is that a lithium-ion battery comprising the lithium iron phosphate obtained by the method had a discharge capacity of 3.38 mAh.
Patent Document 2 discloses a method for producing lithium iron phosphate, and the method comprises placing an aqueous solution containing ferrous sulfate heptahydrate and phosphoric acid in an autoclave and injecting thereto an aqueous solution containing lithium hydroxide, followed by heating the resultant at 50° C., performing nitrogen purge, and raising the temperature to 160° C. for hydrothermal treatment at 160° C.
Patent Document 3 discloses a method for producing a lithium metal phosphate, and the method comprises step a to subject lithium phosphate, a divalent metal salt, and an acidic phosphate source to a reaction in a polar solvent to produce a suspension of phosphate containing the divalent metal, step b to add a basic lithium source to the suspension obtained in step a to obtain precipitate, and step c to convert the precipitate obtained in step b into a lithium metal phosphate. Patent Document 3 discloses that in step a, the pH of the mixture is raised from lower than 2 to the range of 2 to 6, and in step b, the pH of the mixture is raised to the range of 6 to 8. Patent Document 3 also discloses that in step a and step b, the temperature is maintained at 5° C. to 80° C., and in step c, a hydrothermal synthesis reaction is carried out at a temperature of 100° C. to 250° C.